In a switching power-supply device for supply a power source voltage to a digital signal processing LSI (large-scale integrated circuit) or the like, a high load response performance, which suppresses, as far as possible, a fluctuation range of an output voltage due to a digital load dynamically changed, is required. In response to this requirement for load response performance, a ripple converter, in which an error amplifier being a main cause of delay elements is not used and thus load response performance is improved, has been widely used (e.g., see specifications of U.S. Pat. No. 6,583,610 and Japanese Patent Application Publication No. 2008-72809).
Because such a classic ripple converter is a type which detects a ripple voltage of the output voltage to perform a feedback control, a capacitor, as an output capacitor, having a high ESR (Equivalent Series Resistance) or the like is required to obtain a sufficient ripple signal, thereby disturbing downsizing of the system.
Therefore, recently, superimposing a Ramp signal, which is assumed as a ripple due to the ESR of the output capacitor, with a feedback voltage is performed. According to this configuration, a stable operation is possible even if a ceramic capacitor having a low ESR is used as the output capacitor.